In optics, a dispersive prism is a type of optical prism, usually having the shape of a geometrical triangular prism. It is the most widely known type of optical prism, although perhaps not the most common in actual use. Triangular prisms are used to disperse light, that is, to break light up into its spectral components (the colors of the rainbow). This dispersion occurs because the angle of refraction is dependent on the refractive index of a certain material which in turn is slightly dependent on the wavelength of light that is travelling through it. This means that different wavelengths of light will travel at different speeds, and so the light will disperse into the colours of the visible spectrum, with longer wavelengths (red, yellow) being refracted less than shorter wavelengths (violet, blue). This effect can also be used to measure the refractive index of the prism's material with high accuracy. In such a measurement, the prism is placed on the central rotary platform of an optical spectrometer with the incident light beam adjusted such that the refracted beam is at minimum deviation. The refractive index can then be computed using the apex angle and the angle of minimum deviation.
Although the refractive index is dependent on the wavelength in every material, some materials have a much more powerful wavelength dependence (are much more dispersive) than others. Crown glasses such as BK7 have a relatively small dispersion, while flint glasses have a much stronger dispersion (for visible light) and hence are more suitable for use in dispersive prisms. Fused quartz is used in the ultraviolet as normal glasses lose their transparency there.
The top angle of the prism (the upper corner in the accompanying picture) can be chosen to influence the exact dispersion characteristics. However, it is typically chosen such that both the incoming and outgoing light rays hit the surface approximately at the Brewster's angle, so that reflection losses are minimized. An example is the use of this type of prisms in prism compressors for generation of ultrafast laser pulses.
An artist's rendition of a dispersive prism is seen on the cover of Pink Floyd's The Dark Side of the Moon. The iconic graphic shows a coherent ray of white light entering the prism and beginning to disperse. It also shows the spectrum leaving the prism. The graphic is factually inaccurate for two reasons. First, when looking down the long axis of a prism at light passing through the prism at right angles to the long axis, one does not actually see the light inside the prism itself because the beam is moving transverse to the line of sight. One may see back scattering of light into the prism as shown in the photograph above. Although the Pink Floyd graphic indicates accurately that the light disperses upon entering the prism, it errs in suggesting that one could see that internal dispersion.
Second, the Pink Floyd graphic errs in showing the emerging spectrum. When looking at the prism from the end, as the graphic depicts, one would not see the emerging spectrum unless a white reflective surface was provided.
Types of dispersive prism include:
Grisms (grating prisms)
Diffraction gratings may be replicated onto prisms to form grating prisms, called "grisms". A transmission grism is a useful component in an astronomical telescope, allowing observation of stellar spectra. A reflection grating replicated onto a prism allows light to diffract inside the prism medium, which increases the dispersion by the ratio of refractive index of that medium to that of air.
Grating and prism mountings
There are six grating/prism conﬁgurations which are considered to be "classics":
- M. Born and E. Wolf, Principles of Optics, 7 ed. (Cambridge University, Cambridge, 1999), pp. 190–193.
- F. J. Duarte, Tunable Laser Optics (Elsevier Academic, New York, 2003).
- George J . Zissis (1995). "Dispersive prisms and gratings" (pdf) in Michael Bass et al. (eds.) Handbook of Optics. Vol. 2, Ch. 5. McGraw Hill.